As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased as an energy source for the mobile devices. Accordingly, much research into batteries satisfying various needs has been carried out.
Secondary batteries may be classified based on the shape of a battery case of each of the secondary batteries into a cylindrical battery configured to have a structure in which an electrode assembly is mounted in a cylindrical metal container, a prismatic battery configured to have a structure in which an electrode assembly is mounted in a prismatic metal container, and a pouch-shaped battery configured to have a structure in which an electrode assembly is mounted in a pouch-shaped case made of a laminated aluminum sheet.
In addition, secondary batteries may be classified based on the structure of an electrode assembly including a positive electrode, a negative electrode, and a separator. For example, the electrode assembly may be configured to have a jelly-roll (wound) type structure in which long-sheet type positive electrodes and negative electrodes are wound while separators are disposed respectively between the positive electrodes and the negative electrodes, a stacked type structure in which pluralities of positive electrodes and negative electrodes, each having a predetermined size, are sequentially stacked while separators are disposed respectively between the positive electrodes and the negative electrodes, a stacked/folded type structure in which pluralities of positive electrodes and negative electrodes, each having a predetermined size, are sequentially stacked while separators are disposed respectively between the positive electrodes and the negative electrodes to constitute a unit cell, such as a bi-cell or a full cell, and then unit cells are wound in a state in which the unit cells are disposed on a separation film, or another stacked type electrode assembly in which bi-cells or full cells are stacked in a state in which separators are disposed respectively between the bi-cells or the full cells.
In recent years, a lot of interest has been directed to a secondary battery including an electrode assembly that includes bi-cells or full cells, which are easy to manufacture, manufacturing cost of which is low, and which have high structural applicability in response to various shapes of a device, in which the secondary battery is mounted.
Meanwhile, in a case in which a sharp needle-shaped conductor, such as a nail, having high electrical conductivity penetrates into the electrode assembly, the positive electrode and the negative electrode of the electrode assembly are electrically connected to each other by the needle-shaped conductor, with the result that current flows to the needle-shaped conductor, the resistance of which is low. At this time, the electrodes through which the needle-shaped conductor has penetrated are deformed, and high resistance heat is generated due to conducting current in a contact resistance portion between a positive electrode active material and a negative electrode active material. In a case in which the temperature in the electrode assembly exceeds a critical temperature level due to the resistance heat, the oxide structure of the positive electrode active material collapses, and therefore a thermal runaway phenomenon occurs. As a result, the electrode assembly and the secondary battery may catch fire or explode.
In addition, in a case in which the electrode active material or a current collector bent by the needle-shaped conductor contacts the opposite electrode that the electrode active material or the current collector faces, the thermal runaway phenomenon may be further accelerated. These problems may be more serious in a bi-cell including a plurality of electrodes and an electrode assembly including the same.